It would probably have to use a combination of a file page attribute, which lets you specify an image in the page properties menu, and then a custom autonav template, which grabs this image (if it exists) and outputs it. I think there are examples of other people using this in the forum... Does anybody have them?

I had to modify the css a bit since I didn't want a fixed width top level menu. I couldn't find a "nice" solution, the css contains some ugly code now.

Please not that I never built the template to be published. You might run into some problems:
1. You must have two attributes (picture_active, picture_inactive)
2. you must select a picture for all the sites on the top level!
3. I don't know what happens when you use a third level

It would probably be nice to make that menu solid like concrete but I'm too busy to do this right now.. But feel free to use and optimize my code!

I am trying to use the code posted to generate pulldown menus for the navigation but get the following error: Fatal error: Call to a member function getFileRelativePath() on a non-object on line 67. I have put the file menu.php under blocks/autonav/templates and added menu.css to the header.php file under the themes.